The present invention relates to a method of producing a nodular graphite cast iron and, more particularly, to a method of producing a nodular graphite cast iron having a high strength and a high toughness in its as-cast state.
Nodular graphite cast iron exhibits the highest toughness among various cast irons. The nodular graphite cast iron, however, exhibits a tensile strength in the level of around 80 Kg/mm.sup.2 at the highest. It is quite difficult to obtain nodular graphite cast iron having high strength and toughness unless a specific heat treatment consisting of reheating, hardening and tempering is conducted after melting and casting of a specially selected material, or an austempering treatment as disclosed in Japanese Patent Publication No. 48014/78 is conducted.
Needless to say, the reheating of casting once cooled after having been removed from a mold naturally requires considerable energy. Thus, these known methods are not recommended not only from the view point of production cost but also from the view point of energy preservation.
Further, in order to obtain high strength and high toughness stability in the as-cast state, it is necessary to add large amounts of special elements such as Ni, Mo and Cu. These special elements are generally expensive and, hence, increase production costs.
Under these circumstances, such a method has been proposed that, in the course of the cooling after pouring and solidification of the molten metal in the mold down to the completion of A.sub.1 transformation, the removal of casting from the mold is conducted as soon as possible after the solidification to rapidly cool the casting to increase the amount of pearlite in the matrix structure, thereby to improve the mechanical properties. However, the addition of pearlite stabilizers such as Mn, Sn and Cu is also necessary in this method. Even with the addition of these pearlite stabilizers, the cast iron produced by this method cannot have such high strength and toughness as would permit this cast iron to be used in place of forged material. Thus, this method is still unsatisfactory.